Multi antenna technology has attracted attention in wireless communications, because it offers significant increases in data throughput and link range without additional bandwidth or transmit power. In 3GPP (3rd Generation Partner Project) Long Term Evolution (LTE), different multi antenna schemes have been defined for downlink transmission, e.g. in 3GPP TS 36.211: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, V8.7.0. The defined schemes include Transmission Mode 3 (TM3, open loop spatial multiplexing), TM4 (close loop spatial multiplexing), TM7 (single-layer beamforming) and TM8 (dual-layer beamforming), etc. In general, they can be categorized into two kinds: codebook based pre-coding (TM3, TM4) and non-codebook based beamforming (TM7, TM8). Among the transmission modes, TM7 and TM8 are preferred for Time Division-Long Term Evolution (TD-LTE) due to the nature of TDD channel reciprocity, i.e. the property that the signals in uplink and downlink travel through the same frequency band will undergo the same physical perturbations (i.e. reflection, refraction, diffraction, etc. . . . ).
An important requirement for beamforming is to eliminate or compensate different phase shifts as introduced by different antenna ports. FIG. 1 shows a channel model of one antenna port in a TDD system. As shown in FIG. 1, the downlink channel consists of two parts: the transmitter (DL) RF front end path (referred to as RF path hereinbelow) and the wireless channel over air (denoted as Uu in FIG. 1). In the same way, the uplink channel consists of the receiver (UL) RF path and the wireless channel over air. Here the RF path includes RF cable between evolved Node B (eNB) and the antenna, analog devices (like power amplifier (PA), Analog to Digital converter/Digital to Analog converter (ADC/DAC), etc.) and the antenna. In practical systems, all these components on the RF path will introduce different phase shifts between different antenna ports. To perform beamforming e.g. in downlink, it is therefore required to compensate not only the phase shifts between antennas as introduced by the downlink Uu part but also that introduced by the downlink RF path.
The use of channel reciprocity is usually suggested for TDD systems. The eNB could obtain downlink channel state information (CSI) and in turn the phase shift for each antenna port based on the uplink channel estimation. Typically, uplink channel estimation is a Sounding Reference Signal (SRS)-based channel probing as defined in 3GPP TS 36.213: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, V8.7.0. The sounding procedure for each antenna port N is illustrated in FIG. 2. First, the eNB configures the sounding bandwidth, periodicity and hopping patterns through Radio Resource Control (RRC) signaling for each User Equipment (UE). Then the UE period transmits the SRSs based on the signaled configuration, and the eNB estimates the uplink equivalent channel including both Uu and UL RF path based on the received SRSs and obtains the CSI of the uplink equivalent channel.
However, it should be noted that the channel reciprocity is only valid for wireless channel over air (the Uu part). In practical systems, there are noticeable differences between the UL and DL RF paths due to, e.g., different electronic components used in UL/DL RF path, temperature, humidity, etc. The CSI for the uplink equivalent channel including both Uu and UL RF path as obtained by the SRS-based channel probing can not be used as the downlink CSI based on which the phase shift for each antenna port is calculated. In traditional TDD systems, this problem is solved by antenna calibration, which utilize a coupling network (integrated with antenna) to detect CSI of RF path for downlink and uplink separately. With calibration, the eNB may obtain the phase shifts as introduced by the UL/DL RF paths, and in turn calculate the phase shift as introduced by the whole DL channel based on the channel reciprocity.
Unfortunately, coupling network for antenna calibration is not always available. Many TDD operators have invested a lot for 4/8 ports antenna in 2G and 3G. For example, Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) system has been deployed in China, where 8 antennas are used at each base station. Personal Handyphone System (PHS) has been deployed in Japan, where base stations are equipped with 4/8 ports antennas. From migration perspective, the operators require vendors to provide solution for deployment of TD-LTE in the existing 2G (like PHS) system or 3G system (like TD-SCDMA). However, coupling networks are not always available in these existing 2G/3G TDD systems. For example, for the PHS system in Japan, there is no coupling network integrated in antennas, and for the TD-SCDMA system in China, coupling network is only equipped for outdoor coverage antennas, but not for indoor coverage antennas.